This invention relates to absorbents, and to a process for their manufacture, in particular to absorbents suitable for removing halogen-containing contaminants such as hydrogen chloride or chlorine-containing organic compounds from gas streams.
Such absorbents are typically employed as a bed through which the gas stream to be treated is continuously passed: the contaminant is absorbed by the absorbent so that the effluent gas has a relatively low contaminant content. After a period of time, the absorbent becomes loaded with absorbed contaminant until the absorbent bed is unable to reduce the contaminant content to an acceptable level: typically it is desired to have an effluent gas containing less than a specified amount, e.g. 0.1 ppm by volume, of contaminant. When the effluent gas contains an unacceptable proportion of contaminant, xe2x80x9cbreak-throughxe2x80x9d is said to have occurred. It is normally found that, when break-through has occurred, the halide content of the bed is somewhat less than the theoretical maximum: thus while samples taken from bed inlet region may have a halogen content approaching the theoretical maximum, samples taken from the bed outlet region are liable to have a halogen content significantly below the theoretical maximum.
Sodium and zinc compounds are effective absorbents for halogen-containing compounds. It has been proposed in U.S. Pat. No. 3,935,295 to make absorbents from a composition comprising zinc oxide, a basic calcium compound and a binder. It has also been proposed in WO95/22403 to make absorbents by granulating a mixture of sodium carbonate, or bicarbonate, alumina trihydrate, and a binder followed by calcination at below 350xc2x0 C. While such absorbents have a high halogen absorption capacity at low temperatures, e.g. below 150xc2x0 C., at higher temperatures the absorption capacity decreases as a result of decomposition of the active species in the absorbent. It is thus desirable to produce an absorbent that is effective both at temperatures below 150xc2x0 C. and at higher temperatures, e.g. up to 300xc2x0 C.
We have found that particularly effective absorbents can be made from a combination of alkali metal, and/or alkaline earth metal and zinc compounds.
Accordingly the present invention provides shaped absorbent units comprising a calcined intimate mixture of
a) an alumina component selected from alumina and/or hydrated alumina,
b) a zinc component and a basic metal component, said components being oxides, hydroxides, carbonates, bicarbonates and/or basic carbonates and said basic metal component comprising at least one compound of at least one alkali or alkaline earth metal, and
c) a binder,
the basic metal to zinc atomic ratio being in the range 0.5x to 2.5x and the basic metal to aluminium atomic ratio being in the range 0.5x to 1.5x where x is the valency of the basic metal, said granules containing from 5 to 20% by weight of said binder.
Preferably the shaped units have a basic metal content such that, after ignition of a sample of the units at 900xc2x0 C., the sample has a basic metal oxide content of at least 10%, particularly at least 15%, and more particularly at least 20%, by weight.
Basic metal compounds that may be employed include compounds of lithium, sodium, potassium, beryllium, magnesium, calcium, strontium and barium. Preferred compounds are compounds of sodium or calcium, particularly sodium. Particularly preferred alkali or alkaline earth compounds are carbonates and/or bicarbonates. Where an alkali metal compound is used xe2x80x9cxxe2x80x9d=1. Mixtures of alkali metal and alkaline earth metal compounds may be used: in this case the value of xe2x80x9cxxe2x80x9d will depend on the relative atomic proportions of alkali metal and alkaline earth metal.
The zinc component is preferably zinc oxide, carbonate or, particularly, basic carbonate. The basic metal and zinc components may be present at least partially as a mixed salt, such as sodium zinc carbonate and/or basic sodium zinc carbonate.
The shaped absorbent units preferably have an average size in the range 2-10 mm, and preferably at least about 3 mm as a bed of smaller units is liable to present an unacceptable resistance to flow of gas therethrough. Thus an unacceptably high pressure drop is experienced upon passage of the gas through a bed of small units.
The binder may be a suitable hydraulic cement, such as calcium aluminate cement. Alternatively, and preferably, the binder comprises a clay, for example an acicular clay such as attapulgite or sepiolite.
The shaped absorbent units of the present invention may be made by granulating or extruding a mixture of alumina or a hydrated alumina such as alumina trihydrate, basic metal component, zinc component and the binder, in the requisite proportions, and calcining the resultant mixture. Preferably the units are made from a mixture of hydrated alumina, sodium bicarbonate, zinc oxide or basic zinc carbonate, and a clay binder.
Alternatively there may be used a preformed mixed basic metal/zinc salt, e.g. sodium zinc carbonate or basic sodium zinc carbonate, e.g. as obtained by precipitation by the dropwise addition of a solution of sodium carbonate with a solution of a zinc compound such as zinc nitrate under controlled conditions of pH in the range 7-8 and temperature of about 80xc2x0 C., alone, or in admixture with additional zinc and/or sodium carbonates. This mixed basic metal/zinc salt may be mixed with the alumina or hydrated alumina and binder to form the shaped absorbent units.
Where hydrated alumina is used as the alumina component, the calcination results in a substantial increase in the surface area of the absorbents. For these reasons the calcination is preferably effected at temperatures in the range 200-450xc2x0 C., particularly above 240xc2x0 C., and most preferably above 300xc2x0 C. Preferably the calcination temperature is below 500xc2x0 C. to minimise reaction of the basic metal compound and the alumina: thus alkali or alkaline earth metal aluminates have lower absorption capacity.
The shaped absorbent units preferably have a BET surface area of at least 10 m2/g, particularly above 50 m2/g, and most preferably above 90 m2/g.
By the term granulation we mean mixing the powdered ingredients, including the binder, with a little wetting agent such as water, in an amount that is insufficient to form a slurry, and forming the resultant mixture into aggregates, generally of approximate spherical configuration. Such granulation techniques are well known in the art.
As an alternative to granulation, the composition may be formed into extrudates, for example using a pellet mill, for example of the type used for pelleting animal feedstuffs, wherein the mixture to be pelleted is charged to a rotating perforate cylinder through the perforations of which the mixture is forced by a bar or roller within the cylinder. The resulting extruded mixture is cut from the surface of the rotating cylinder by a doctor knife positioned to give pellets of the desired length. It will be appreciated that other extrusion techniques may be employed.
It is preferred to employ alumina trihydrate, rather than alumina, since granulation or extrusion of alumina-containing compositions tends to present difficulties.
In order to make shaped units of adequate strength it is desirable to employ the ingredients in a finely divided form. Typically the ingredients have an average particle size in the range 1-20 xcexcm, preferably in the range 5-10 xcexcm.
Where an alkali metal compound is used, during the calcination step, it is believed that there is formed a highly dispersed alkali metal/zinc composite, probably an intimate mixture of alkali metal carbonate and zinc oxide, that is uniformly distributed over the alumina substrate. It appears that while zinc oxide is an effective chloride absorbent, alkali metal carbonates are less effective. For this reason the alkali metal to zinc atomic ratio of the granules should be below 2.5. When employing sodium compounds as the alkali metal component and the absorbent is used for absorption of hydrogen chloride, we have observed that a sodium zinc chloride Na2ZnCl4 is formed: this species has been identified by X-ray diffractometry on absorbents that have been used for absorption of hydrogen chloride. It is evident by the dry, free-flowing nature of the fully chlorided shaped absorbent units that the formation of this compound does not give rise to any of the deleterious effects associated with moisture absorption such as caking, pressure drop, and difficulties in discharge of spent absorbent. However, if the alkali metal to zinc ratio of the absorbent is too small, the granules become sticky during use, giving rise to caking of a the bed of absorbent units with the consequence of the bed exhibiting an unacceptable increase in the resistance of gas flow therethrough. For this reason the alkali metal to zinc atomic ratio should preferably be above 0.8. It is preferred that the alkali metal to zinc atomic ratio is in the range from about 0.8 to 22.
The absorbent granules of the invention may be used at temperatures ranging from 10 to 300xc2x0 C. and at any convenient pressure, for example atmospheric to 100 bar abs. They are of particular utility as guard beds to absorb chloride ions from gas streams, e.g. to avoid corrosion problems during subsequent processing of the gas stream and/or to avoid poisoning of downstream catalysts, particularly copper containing catalysts such as low temperature shift catalysts or methanol synthesis catalysts. Thus the granules may be disposed as a bed adjacent the inlet of a bed of low temperature shift or methanol synthesis catalyst. They may also be of utility in removing halogen-containing organic compounds from gas streams.